More galaxies in the Local Volume imaged in H-alpha
aa r X i v : . [ a s t r o - ph . C O ] O c t More galaxies in the Local Volume imaged in
H α
Igor D. Karachentsev and Serafim S. KaisinSpecial Astrophysical Observatory, Russian Academy of Sciences, N. Arkhyz, KChR,369167, Russia [email protected]
Received ; accepted 2 –
ABSTRACT
We have carried out an Hα flux measurement for 52 nearby galaxies as part ofa general Hα imaging survey for the Local Volume sample of galaxies within 10Mpc. Most of the objects are probable members of the groups around Maffei 2/IC342, NGC 672/IC 1727, NGC 784, and the Orion galaxy. The measured Hα fluxescorrected for extinction are used to derive the galaxy star formation rate (SFR).We briefly discuss some basic scaling relations between SFR, hydrogen mass andabsolute magnitude of the Local Volume galaxies. The total SFR density in thelocal ( z = 0) universe is estimated to be (0 . ± . M ⊙ yr − M pc − .
1. Introduction
Systematic measurements of Hα fluxes in nearby galaxies within a fixed distance isone of the major techniques for determining the star formation rate (SFR) in the localuniverse. The presence of dwarf galaxies with extremely low luminosities in the LocalVolume which are usually invisible at large distances, provides a unique opportunity forresearching the SFR of a galaxy depending on its luminosity, structure, gas mass, anddensity of its environment in the broadest possible range of these characteristics. Thus it isessential that the observational program contains objects of all types and sizes in order toavoid the observational selection, distorting the interpretation of results.Kraan-Korteweg & Tammann (1979) proposed to view an exemplary sample of179 nearest galaxies with distances within 10 Mpc (the so-called LV sample). Later,Karachentsev (1994) replenished it with galaxies discovered from new redshift surveys.The updated sample constituted 226 galaxies. Later, Karachentseva & Karachentsev(1998,2000) and Karachentseva et al. (1999) undertook the all-sky search for nearbygalaxies using plates of the POSS-II and ESO-SERC survey. A large number of newnearby dwarf galaxies of low surface brightness were found. A limiting magnitude of thesurvey was B ≃ m providing an essential completeness of the LV sample up to absolutemagnitude M B ≃ − . m within a distance of 8 Mpc. Their radial velocities were measuredby Huchtmeier et al. (2000, 2001) during a subsequent HI-survey. Distances to many ofthe LV galaxies were first measured with high accuracy on the Hubble Space Telescope(HST) using the tip red giant branch (TRGB) method. The results of these internationalefforts were summarized in the Catalog of Neighboring Galaxies (CNG) (Karachentsev etal., 2004), consisting of 451 galaxies with distance estimates within D <
10 Mpc.Different surveys of large areas of the sky in the optical and radio bands: SDSS(Abazajian et al. 2009), 2dF (Colless et al. 2001), 6dF (Jones et al. 2004), HIPASS (Zwaanet al. 2003), and ALFALFA (Giovanelli et al. 2005) have led to an essential increase ofthe Local Volume sample. The second version of the CNG (Karachentsev et al. 2011, inpreparation) contains N ≃
790 galaxies, i.e. more than four times that of the original listby Kraan-Korteweg & Tammann (1979). Such a representative sample, being registered 3 –in the Hα emission line, provides a detailed picture of star formation in the Local Volumeduring the recent time interval of ∼
10 Myr.Measurements of Hα fluxes in nearby galaxies were performed by many authors. Theirinterest was normally fixed on objects of a certain type, for example, on spiral or irregulargalaxies. A synopsis of publications, where the number of measured Hα fluxes for the LocalVolume galaxies was not less than 10, is presented in Table 1. The first column of the tablecontains a link to the paper, the second contains the number of LV galaxies imaged in Hα ,and the third column reflects the nature of the sample (the morphological type of galaxiesor their affiliation to a fixed group). The last row of the table summarizes the numberof LV galaxies, which were observed in our program (Karachentsev et al. 2005; Kaisin &Karachentsev 2006, 2008; Kaisin et al. 2007; Karachentsev & Kaisin 2007), including theresults of this paper. Unlike all previous observational programs, our Hα survey of LVgalaxies does not imply any selection of objects by morphological type, or their affiliationto a group. This has led to some unexpected results, in particular, to the detection ofcircumnuclear emission in isolated E/S0 galaxies (Moiseev et al. 2010).As follows from the data above, so far there are 692 measurements of the Hα fluxavailable for 435 LV galaxies, i.e. a lot of galaxies were observed independently by differentauthors, which enables an estimate of the external error of the flux measurement. Morethan half of the Hα data were obtained within the framework of two programs: Kennicuttet al. (2008) and our survey.The degree of completeness of the Local Volume galaxy collection currently availableremains quite ambiguous. New sky surveys reveal new nearby galaxies of both low and highsurface brightness. Refinement of individual distances to galaxies leads to their inclusion orexclusion as LV members. Among ∼
790 galaxies that are currently listed in the LV, thereare objects with absolute magnitudes M B ranging from − m to − m , i.e. their luminositiesvary by more than seven orders of magnitude. Figure 1 presents the distribution of 572galaxies situated within 8 Mpc according to their B-band absolute magnitude. The shadedareas on the left and right panels indicate numbers of galaxies that have been observedin Hα and HI , respectively. The median absolute magnitude of the LV galaxies is − m .Measurements of the Hα flux are carried out for 355 galaxies or 62% of the sample. Asone can see, the current Hα survey is almost complete on the bright half of the luminosityfunction, i.e. up to M B ≃ − m . For a comparison note that the completeness of the LVgalaxy survey in the neutral hydrogen line HI is much higher, reaching 88%. A comparisonof these panels indicates the need to speed up the survey of nearby galaxies in the Hα lineto have a sufficiently complete picture of the SFR diversity in them.
2. Observations and image processing
Below we present a survey of 52 nearby galaxies, most of which were observed inthe Hα line for the first time. Some of these galaxies are likely members of the closest 4 –neighboring group Maffei2/IC342, poorly studied due to its location in the region of strongabsorption in the Milky Way; the other part is a mixture of field galaxies and members ofother small groups, in particular, around NGC 672, NGC 784, and in the Orion region.CCD images of galaxies in the Hα -line and in the continuum were obtained duringobserving runs from 2005 to 2009. An average seeing was 1 . ′′ . All the observations wereperformed in the Special Astrophysical Observatory of the Russian Academy of Sciences(SAO RAS) with the BTA 6-m telescope equipped with the SCORPIO focal reducer(Afanasiev et al. 2005). A CCD chip of 2048 × ′ with a scale of 0.18 ′′ /pixel. The images in Hα +[NII] were obtained viaobserving the galaxies though a narrow-band interference filter Hα (∆ λ =75˚A) with aneffective wavelength λ =6555˚A. In order to remove the stellar continuum contribution tothese images, we also observed the same ields with two medium-band filters situated onboth sides from Hα : SED607 with λ =6063˚A, ∆ λ =167˚A, and SED707 with λ =7063˚A,∆ λ =207˚A. Typical exposure times were 2 × Hα and 2 × s in the continuum.Since the range of radial velocities in our sample is small, we used the same Hα filter for allthe observed objects. Our data reduction followed the standard practice and was performedwithin the MIDAS package. For all the data, bias was subtracted and the images wereflat-fielded by twilight flats. Cosmic particles were removed and the sky background wassubtracted. The next operation was to spatially align all the images for a given object.Then the images in the continuum were normalized to Hα images using 5–15 field starsand were subtracted. Hα fluxes were obtained for the continuum-subtracted images, usingspectrophotometric standard stars from Oke (1990) observed during the same nights as theobjects. The investigation of measurement errors, brought in by the continuum subtraction,flat-fielding, and scatter in the zeropoints, has shown that they have typical values within15%. We did not correct Hα fluxes for the contribution of the [NII] lines, because it islikely to be small for the majority of low-luminosity galaxies in our sample. For instance,according to Equation (1B) in Kennicutt et al. (2008), a typical galaxy in our sample withthe median absolute magnitude M B = − m has a [NII]/ Hα ratio of 1/20 which is lowerthan the accuracy of our measurements. Hα fluxes and SFRs The measured integrated flux of a galaxy in the Hα +[NII] lines, calibrated according toOke’s spectrophotometric standards (and noted as F ( Hα )), was expressed in units of (erg × cm − × sec − ). In each case, we tried to take into account not only the sum of emissionknots of the galaxy but also its diffuse emission background insofar as it was not distortedby the background subtraction errors, significant in large apertures. Due to the width of thefilter used, the measured galaxy fluxes are also containing emission in the neighboring linedoublet [NII]. However, according to Kennicutt et al. (2008), relative contribution of thedoublet for the majority of galaxies is small, especially for dwarf galaxies. The measured F ( Hα ) flux is then corrected for light absorption in the Milky Way A B (MW) using a 5 –technique by Schlegel et al. (1998), and for internal extinction in the galaxy itself A B (int),defined as A B (int) = [1 . . V m − . × log( a/b ) , (1)if V m > . − , and A (int)=0 otherwise. This ratio incorporated the known fact(Verheijen 2001) that internal extinction depends not only on the inclination of the galaxy,expressed in terms of its apparent axial ratio a/b , but also on its luminosity, an indicator ofwhich according to Tully & Fisher (1977) is the amplitude of its rotation velocity V m . Here,the quantities a/b , V m , and A B (int) are taken from Tables 1 and 4 of the CNG. Absorptionin the Hα line was adopted as proportional to the absorption in the B -band: A ( Hα ) = 0 . A B (MW) + A B (int)] . (2)Following Gallagher et al. (1984), we calculated the integrated star formation rate inthe galaxy as SF R ( M ⊙ /year ) = 1 . × × F c ( Hα ) × D , (3)where D is the distance to the galaxy, expressed in Mpc. The validity of the lineartransition (3) from the flux F c ( Hα ) to the SFR has recently been a subject of criticalreviews. Pflamm-Altenburg et al. (2007) and Pflamm-Altenburg & Kroupa (2009)exhaustively argued that the canonical relation (3) underestimates the SFR value in dwarfgalaxies, and the weaker the luminosity of the galaxy is, the stronger the difference is.In dwarf systems with absolute magnitude M B ∼ − , − m , an underestimation of theSFR value may reach one to two orders of magnitude. To be pricise, in what follows weconserve the SFR estimates made under the canonical relation (3). We divided the galaxieswe observed into two categories: 19 galaxies belonging to a nearby association aroundMaffei2/IC342 and 33 general field galaxies that include both isolated galaxies and membersof multiple systems. The galaxies Maffei1, Maffei2, and their companions are obscuredfrom us by dust clouds, which create a strong and heterogeneous absorption, reaching A B ∼ m − m . This circumstance considerably hampers the measurements of F ( Hα ) fluxesand increases the errors. Moreover, the magnitude of absorption A B itself is determined atlow galactic latitudes | b | < ◦ with a high uncertainty.The images of 19 galaxies from the Maffei2/IC342 group and 33 general field galaxiesare represented as a mosaic in Figures 2 and 3, respectively. The left and right images ofeach galaxy correspond to the sum and difference of the images that have been exposed inthe Hα and in the continuum. The image scales are shown by the horizontal bars equal to1 arcmin, and the north and east orientation is indicated in the corner by arrows.The main characteristics of the observed galaxies are listed in Tables 2 and 3, thestructures of which are identical. The table columns contain: (1) — galaxy name, (2) — 6 –equatorial coordinates for epoch J2000.0, (3) — morphological type from de Vaucouleursdigital classification, (4) — radial velocity relative to the Local Group centroid, (5) —distance to the galaxy presented in the CNG catalog including new data from Tully et al.(2006) and Karachentsev et al. (2006), (6) — absolute magnitude in the B band from theCNG, corrected for internal and external absorption, (7) — logarithm of the hydrogen massof the galaxy, log( M HI /M ⊙ ) = log F HI + 2 log D + 5 .
37, where F HI is the flux in the HI line(in J y km s − ), and D in Mpc, (8,9) — the observed and corrected flux in the Hα line, and(10) — integrated star formation rate in the galaxy calculated from the canonical relation(3).
4. Comments on individual objects in the Maffei group
The structure and kinematics of the binary association of galaxies around the majorspirals Maffei2 and IC342 as subgroup centers were considered by Karachentsev et al.(2003a). Most of the galaxies presented in Table 2 are physical companions of either Maffei2or IC342, judging from their radial velocities.
KKH5, KKH6.
Both dIrr galaxies are peripheral members of the association, thathave probably not yet reached the virialized region around Maffei2. Their distances aremeasured with an accuracy of ∼
10% from the TRGB detected on the images derivedwith the Hubble Space Telescope (Karachentsev et al. 2003b, 2006). Both galaxies showemission knots, which are more diffuse in the case of KKH6.
Cas1 = KK19.
This dIrr galaxy is located in the zone of strong absorption ( A B = 4 . m ),which remains yet inaccessible for determining the distance via the TRGB. Its distance asa companion of IC 342 is adopted at 3.3 Mpc. The Hα image reveals many compact HIIregions some of which are circular-shaped. KKH11, KKH12, MB1 = KK21.
All the three irregular galaxies are companions ofMaffei2. Each galaxy demonstrates the presence of emission knots and filaments with thevalues of the total Hα flux close to one another. Maffei1.
Maffei1 is an elliptical galaxy, the distance to which is estimated by Fingerhutet al. (2003) from the central dispersion of radial velocities. Its angular sizes, correctedfor absorption A B = 5 . m , extend beyond our image frame, which created problems withbackground subtraction. Apart from the residues of overexposed star images, there are novisible traces of emission regions on the body of the galaxy. In the table we indicate theupper limit of its possible Hα flux. Maffei2.
This barred spiral (Sbc) galaxy also extends beyond our image frame. Basedon its apparent K s magnitude (5 . m ) from the Two Micron All Sky Survey (2MASS) andfrom the width of the HI line (305 km s − ), we estimated its distance as 3.1 Mpc using theinfrared Tully–Fisher relation M K = − .
35 lg( W ) + 0 .
75. We obtained the total Hα fluxof Maffei2 assuming that the Hα profile of the galaxy reproduces its brightness profile in 7 –the K –band whereas about 50% of the luminosity appeared to be outside of our frame. Dwingeloo2.
Dwingeloo2 is an irregular galaxy in the region of strong absorption( A B = 5 . m ). A rather bright star is projected on it. The image of this star is likelyscreening a significant part of the galaxy’s Hα flux. MB3 = KK22.
For this dIrr galaxy, we estimated only the upper limit of its Hα flux. Dwingeloo1 = KK23 = Cas2.
This is a spiral SBbc-type galaxy, prone to strongabsorption ( A B = 6 . m ). For it we adopted the mean distance of the group 3.0 Mpc,although an individual assessment of the distance from the infrared Tully-Fisher relationgives (at K s = 8.82 and W = 187 km s − ) a distance of 4.7 Mpc. Approximately 20% ofthe luminosity of the galaxy is beyond the range of our image. KK35.
This object looks like an isolated spot of low surface brightness at the distanceof 16 arcmin from the center of IC 342. It can be a dIrr galaxy in the process of mergingwith the giant spiral IC342 or a stellar association on the outer spiral pattern of the galaxy.Its distance, 3.16 Mpc, determined via the TRGB, is consistent within errors with thedistance of IC342 itself, 3 .
28 Mpc, defined via the Cepheids. As the Hα image shows, KK35is in a state of very active star formation. UGCA86.
This is an Sm/Im galaxy with a bright association of blue stars (VIIZw9) onthe SE side. Its distance, 2.96 Mpc, measured via the TRGB (Karachentsev et al. 2006),confirms the affiliation of UGCA86 to the IC 342 companions. Compact emission knots andfilaments are scattered throughout the galaxy disk, but more than half of the integrated Hα flux comes from a powerful site of star formation, VIIZw9. CamA = KK41, CamB = KK44.
Both are dIrr galaxies of low surface brightness thatshow the presence of a blue stellar population in the images obtained with the WFPC2on the HST (Karachentsev et al. 2003a). Both galaxies show small compact knots and aweak diffuse emission component in the Hα line, which indicates a sluggish process of starformation in these systems. NGC1569.
Along with M82 and several Markarian galaxies, NGC1569 belongs to theobjects of the Local Volume with the most active star formation per luminosity unit of thegalaxy. An abundance of arc-like emission filaments in the periphery makes the galaxyresemble a crab. Due to significant absorption ( A B = 3 . m ) the distance to NGC1569 haslong remained uncertain. Recently Grocholski et al. (2008) determined its distance as 3.36Mpc using the TRGB method. UGCA92.
This dIrr galaxy is located in the sky in the vicinity of the previous galaxy.Radial velocities and distances from the observer of UGCA92 and NGC1569 are alsoclose. These galaxies can form a bound pair on the outskirts of the group around IC342,reminiscent of the famous pair of NGC 147 and NGC 185, which are dwarf galaxies in theneighborhood of M31. In the UGCA92 body there are groups of bright emission knots, andtwo arcs interlocking in the northern part of the galaxy. 8 –
NGC1560, UGCA105.
These are two galaxies of late Sd and Sm types, oriented atdifferent angles to the line of sight. Their Hα fluxes are also approximately equal. Theircharacteristic features are ring-shaped HII regions, apart from which there are manycompact emission knots.
5. Comments on individual objects in the field
NGC404.
This is the nearest isolated lenticular galaxy of moderate luminosity. In itscentral part small dust clouds are visible. Del Rio et al. (2004) found an extended HI shellaround NGC404, and ultraviolet observations with GALEX have identified a ring-shapedstructure of young stars (Thilker et al. 2010). On our Hα image one can see fairly brightemission in the circumnuclear region of the galaxy, as well as separate emission knots,scattered in the periphery. The integrated Hα flux of the galaxy, shown in Table 3, does nothave any contribution of distant emission knots in the HI region shell. Recently, Williamset al. (2010) presented HST/WFPC2 observations across the disk of NGC404 and studiedits star formation history in detail. AGC112521.
This dIrr galaxy of low surface brightness was detected in the “blind”HI survey ALFALFA, performing with the Arecibo radio telescope (Saintonge et al. 2008).Our Hα image of the galaxy shows only one compact emission knot, which was prevoiuslyspotted by Zitrin & Brosch (2008). KK13, KK14, KK15.
These are dwarf irregular companions of the spiral galaxyNGC672, discovered by Karachentseva & Karachentsev (1998), and detected in the HIsurveys by Huchtmeier et al. (2000) and in the ALFALFA. All three objects have severalcompact emission knots, which were also noted by Zitrin & Brosch (2008).
IC1727, NGC672.
These are a pair of spiral galaxies of late types (Sm, Sd). Theirdistance, 7.2 Mpc, was estimated by I. Drozdovsky (private communication) from theluminosity of brightest stars. It is the distance we ascribed to the four remaining membersof the NGC672 group: AGC112521, KK13, KK14 and KK15. Hα images of both spiralsexhibit many HII regions, typical for Sm and Sd galaxies. Integrated Hα fluxes of IC1727and NGC672 are in good agreement with the fluxes, previously measured by Kennicutt etal. (2008), but are nevertheless significantly (by two to three times) higher than the fluxes,given by Zitrin & Brosch (2008). UGC1281, KK16, KK17.
These three dwarf galaxies form a group along with abrighter Sd galaxy NGC784. Their distances were measured by Tully et al. (2006) using theTRGB method. Zitrin & Brosch (2008) imaged all four galaxies in Hα . Their descriptionsof emission components in these galaxies are consistent with what we see in Figure 3.However, in the case of KK16, where only a faint diffuse emission is visible, the Hα fluxmeasured by us is five times higher than that obtained by Zitrin & Brosch (2008). A reasonfor this difference remains unclear to us. 9 – UGC1703 = KKH9.
This is a dwarf spheroidal galaxy at a distance of 4 . ± . Hα image of UGC1703 does not show any signs of emission. Table 3 indicates the upperlimit of its Hα flux. NGC855.
This isolated elliptical galaxy is similar to NGC404 in terms of luminosityand hydrogen mass. Its distance, 9.73 Mpc, was determined by Tonry et al. (2001)from fluctuations of surface brightness. Wallington et al. (1988) noted the presence of aring-shaped HI shell around it, which is atypical for E galaxies. Our snapshot of NGC855in the Hα line reveals bright emission in the circumnuclear area, external parts of whichare extended in the polar directions. To understand the nature of this object with hybridproperties of E and S galaxies, it is necessary to investigate its kinematics in the HI and Hα lines. AGC122226= KUG0243+275.
This isolated blue compact galaxy was detected in theHI line in the Arecibo survey (Saintonge et al. 2008). The Hα image shows active starformation, concentrated in several knots within the central region of this galaxy. AGES.
This dIrr galaxy was discovered as a result of the “blind” AGES survey in theHI line of the vicinity of an isolated galaxy NGC1156 (Minchin et al. 2011). Judging fromits radial velocity, the AGES object is a dwarf companion of NGC1156 at the projectiondistance of ∼
80 kpc, if we adopt for it the same distance as that of NGC1156, 7.8 Mpc. Ourimage of AGES in the Hα line reveals two diffuse HII regions with a total star formationrate of ∼ × − M ⊙ /year. KKH18.
KKH18 is an isolated dIrr galaxy, the distance to which, 4.43 Mpc, isdetermined via the TRGB (Karachentsev et al. 2003b). KKH18 and UGC1703 are possiblyforming the eastern extension of a filament of dwarf galaxies, the dense part of which is alsohosting the group around NGC784. A compact HII region and weak diffuse emission in thecenter are visible on the body of KKH18.
UGC2773.
This is an isolated BCD galaxy in the region of significant ( A B = 2 . m )absorption. Its Hα image demonstrates many compact HII regions, as well as a considerablediffuse emission. The distance to UGC2773 is estimated by us simply from the radialvelocity taking the Hubble parameter of H = 72 km s − Mpc − . Of course, in the localuniverse, peculiar motions can dominate over the systematic Hubble component. However,at present there is no generally accepted model that describes the local peculiar velocityfield well right taking into account the Virgocentric infall and the Local Velocity Anomaly(see details in Tully et al. 2008). UGC2905.
UGC2905 is an isolated dIrr galaxy on the southern part of which abackground spiral neighbor is projected. The distance to UGC2905, 5.8 Mpc, is estimatedfrom the brightest stars (Georgiev et al. 1997). Its Hα image reveals several compact anddiffuse HII regions. 10 – UGC 3303.
This is an isolated Sm galaxy with a bright star projected on its centralpart. The distance of UGC 3303 is estimated as 7.2 Mpc from the brightest stars (Makarova& Karachentsev 1998). It may be located in the periphery of a scattered association ofgalaxies, the brightest member of which is the Orion galaxy. The Hα image reveals a lot ofsmall HII regions scattered across the disk of the galaxy. KK49 = CGCG422–003.
This is a BCD galaxy in the Orion complex. Its distanceis evaluated from the radial velocity. The Hα image of the KK49 body looks granulatedbecause of the tightly located emission knots. Orion, An0554.
These are two galaxies (Sm and dIrr) located in the Orion complex inthe region of significant galactic absorption. Their distances (6.4 and 5.5 Mpc, respectively)are determined by Karachentsev & Musella (1996) from the luminosity of brightest stars.The HII regions, more abundant in the Orion galaxy in accordance with its morphologicalSm type, are visible on their Hα images. Recently, Cannon et al. (2010) carried out Hα and VLA HI observations of the Orion galaxy and found the rotating HI disk extending faroutside the optical boundary of the galaxy. HIZOA J0630+08.
This HI source detected in the survey by Donley et al. (2005) islocated in a dense stellar region of the Milky Way at the galactic latitude b = − . ◦ . Onthe POSS-II blue and red images, not a single galaxy is seen within the radio telescopebeam ( ∼ ′ ). Our Hα image does not show an optical counterpart to this radio sourceeither, which is most likely a dIrr galaxy of low surface brightness, weakened by absorption( A B = 2 . m ). UGC3476, UGC3600, UGC3698.
These are three isolated dIrr galaxies, the distancesto which are found from the brightest stars (Makarova & Karachentsev 1998). Each oneof them demonstrates the presence of active star formation sites which is characteristic ofisolated irregular galaxies.
UGC3755.
This is a dIrr galaxy, the distance to which is measured by Tully etal. (2006) applying the TRGB. Its image in Hα indicates active star formation, mostpronounced in the western part of the galaxy. DDO47, KK65 = CGCG087–033.
These two galaxies are dwarf galaxies, forming anisolated pair with a difference in radial velocities of only 6 km s − . The distances measuredby Tully et al. (2006) via the TRGB confirmed a physical relationship of these galaxies. Inboth galaxies there are visible compact emission regions. KK69, KK70.
KK69 and KK70 are two dwarf companions of the spiral galaxyNGC2683. Both have low surface brightness. An irregular dwarf, KK69, is characterized bya very narrow HI emission with a line width of 16.5 ± − (Huchtmeier et al. 2003).Our Hα image shows a very red or emission star-like object within its optical boundaries.The nature of this object can be clarified by spectral observations. The dwarf spheroidalsystem KK70 lacks any signs of Hα emission. 11 – NGC2787, NGC4600.
These are two isolated lenticular galaxies, the distances to whichare determined from surface brightness fluctuations (Tonry et al. 2001). In both cases thecentral parts of galaxies are over-exposed, which makes the assessment of the flux in Hα somewhat uncertain.
6. External comparison of Hα fluxes The accuracy of measurement of the Hα flux of a galaxy, and of the SFR valuedetermined from it, depends on many factors. If variable atmospheric conditionswere successfully monitored during the observations by regular calibration using thespectrophotometric standards, the main source of errors for the F ( Hα ) values is inaccuratesubtraction of the sky background on the obtained images. For compact starburst galaxiesthese errors are negligible, but for the galaxies with weak and diffuse Hα emission, sucherrors appear to reach ∼ − − F ( Hα ) as ∼ ± .
06 dex in the logarithmicscale. However, this internal assessment needs to be subject to an independent externalaudit.Among the 52 galaxies we observed there are 12 objects, in which the Hα fluxes weremeasured by Kennicutt et al. (2008), and 11 galaxies observed in Hα by Zitrin & Brosch(2008). The data on Hα fluxes in these galaxies are presented in Table 4. In the case ofKennicutt et al. (2008), we also cite the internal flux measurement errors indicated bythem.A comparison of our lg F ( Hα ) values with the data by Kennicutt et al. (2008) yieldsthe mean square difference σ (∆ lg F )= 0.09 and the average difference h lg F KK − lg F Ken i =+0 . ± .
03, which indicates a good agreement of independent measurements. The internalerror of our measurements that we have estimated as σ (lg F ) = 0 .
06 is approximately thesame as in Kennicutt et al. (2008) (0.058), and their quadratic sum reproduces well themean square difference σ (∆ lg F ) = 0.09. Note, however, that the agreement of our datawith the Hα fluxes, published by Zitrin & Brosch (2008) turned out to be much worse: σ (∆ lg F ) = 0 .
34 and h lg F KK − lg F NB i = +0 . ± . Hα flux of a galaxy to the SFR value is accompaniedby additional errors, which are usually systematic. These factors include: the contributionof the emission line [NII] in the total registered Hα + [NII] flux, different methods ofcorrection for internal absorption in a galaxy, uncertainty of the Galactic absorption valueaccording to Schlegel et al. (1998) at low latitudes, underestimation of the diffuse emissioncomponent of very low surface brightness, and underestimation of possible HII regionsin the distant periphery of a galaxy (the case of NGC404). Finally, as noted above, thetransformation of F ( Hα ) into the SFR via the linear relationship (3) can significantly (by 12 –one to two orders of magnitude) underestimate the true star formation rate due to thesimplistic notions on the initial stellar mass function in dwarf systems (Pflamm-Altenburg& Kroupa 2009).
7. Some basic scaling relations
Estimates of the global star formation rate are currently obtained for 435 LV galaxies.As it is noted by many authors (Karachentsev & Kaisin 2007; James et al. 2008; Thilkeret al. 2007; Lee et al. 2009) that SFR value correlates with the integrated luminosity of agalaxy, its morphological type, color index and hydrogen mass. The data on the dependenceof an SFR of a galaxy on its environment are rather contradictory (Hunter & Elmegreen2004; James et al. 2004), but the prevailing view is that such a dependence, if it exists, isweak, i.e. the process of star formation in the galaxy is more likely driven by its internalstate than by external factors. Nevertheless, there are well-known cases where a closeinteraction or merger of galaxies leads to a spectacular burst of star formation or, the otherway around, a passage of a dIrr galaxy close to a massive spiral suppresses star formationin the dwarf system due to gas stripping from its shallow potential well.Figure 4 represents a relation between the global star formation rate and blue absolutemagnitude for 435 LV galaxies. The galaxies of different morphological types are shownby characters of different colors. Empty symbols with arrows mark the instances whenonly the upper limit of the SFR of a galaxy is known, determined from Equation (3). Thestraight line in the figure corresponds to the constant specific SFR per luminosity unit,
SSF R = 0 . × − M ⊙ yr − L − ⊙ . Evidently, dwarf galaxies are systematically locatedbelow this line. According to Pflamm-Altenburg & Kroupa (2009) their deviation from thelinear relation is leveled if the transition from the measured Hα flux to the SFR is madein the light of modern ideas on the initial stellar mass function in dwarf systems. Thegalaxies from Tables 2 and 3 do not noticeably stand out among the rest of the objects.A distinctive feature of the { SF R, M B } diagram is the presence of a rather sharp upperboundary, SSF R = 4 . × − M ⊙ yr − L − ⊙ , which is mainly traced by dIrr, BCD, andSm–Sc galaxies. Of the galaxies listed in Tables 2 and 3, UGC2773, KK35 and NGC1569present examples of such cases. The existence of a critical upper value for the SSFR isobviously an important universal parameter, characterizing the process of conversion of gasinto stars.The distribution of LV galaxies by SFR values and hydrogen mass is presented in Figure5, where the symbol designations are the same as in Figure 4. The dashed line correspondsto the constant specific SFR, related to the unit of hydrogen mass. The solid line shows asteeper dependence of SFR ∝ M / HI , followed by individual emission complexes inside thegalaxies (the Schmidt–Kennicutt law). According to Pflamm–Altenburg & Kroupa (2009),recalculation of SFR values for dwarf galaxies with the view of a more accurate initialstellar mass function decreases the regression slope in Figure 5 from 1.5 to 1.0. 13 –To describe the evolutionary status of various samples of galaxies, Karachentsev& Kaisin (2007) suggested to use the diagnostic ”past–future” diagram, where thedimensionless and distance-independent parameters P = log { [SFR] × T /L K } , F = log { . M HI / ([SFR] × T ) } (4)show which part of the observed stellar mass of the galaxy can be reproduced at thenow observed SFR during the cosmological time T , and for how long the star formationcan continue there with the present gas reserves of M gas = 1 . M HI . Here the factor 1.85gives a correction for the average abundance of helium and molecular gas in the galaxy(Fukugita & Peebles, 2004). For the P parameter in expression (4), we use a knownfact that the infrared K-band luminosity L K of a galaxy reproduces its stellar mass at M ∗ /L K = 1 M ⊙ /L ⊙ (Bell et al. 2003; Karachentsev & Kut’kin 2005). We adopted K s -bandmagnitudes for 122 LV galaxies from the 2MASS survey (Jarrett et al. 2003). For theremaining objects we transferred their B-magnitudes into the K s ones, using the empiricalrelations between the average color index h B − K i and the morphological type of a galaxy,discussed by Jarrett et al. (2003) and Karachentsev & Kut’kin (2005): h B − K i = 4 .
10 for T ≤ h B − K i = 4 . − T / T = 3 − h B − K i = 2 .
35 for T = 9 , . The distribution of 435 LV galaxies on the ”Past–Future” plane is presented in Figure6, where galaxies of different types, (E, S0, dSph), (Sa, Sab, Sb, Sbc), (Sc, Scd, Sd, Sdm,Sm), and (Irr, BCD), are given in four separate panels. As above, open symbols with arrowsindicate objects with only the upper limit of SFR or HI-flux. Here, we omitted 41 galaxieswith the upper limit of both SFR and HI-flux because of their uncertain position on theF–scale.It is easy to see that the galaxies of different morphological types occupy differentregions on the { P, F } plane, demonstrating the expected evolutionary segregation. Theevolutionary trend according to galaxy types is also reflected in the data of Table 5. Itscolumns indicate: (1) — morphological type, (2) — number of galaxies of this type in theLV with measured SFRs, (3,4) — median values of the P and F parameters. We can drawthe following conclusions from these data.1. The current SFRs in the E, S0, and dSph galaxies can reproduce only about 2% oftheir stellar mass, therefore in the past their average SFR was significantly higher.Typical gas reserves in the E, S0, and dSph galaxies are rather uncertain, and theirtypical gas consumption timescale remains uncertain too.2. According to the median parameters P and F , the spiral galaxies of early types,dominated by the bulges, have already passed the peak of their evolution. The past 14 –SFR was an order of magnitude higher than the present one, and the current gasreserves can support the SFRs during merely 28% of the cosmological timescale.3. In disk-like galaxies of the late Sc–Sm types, the current SFR is only slightly lowerthan it was in the past. The resources of gas in Sc–Sdm galaxies are supplying theirobserved SFRs during almost another Hubble time.4. The population of Irr and BCD galaxies had almost the same mean SFR in thepast, as it does now. Their gas reserves are sufficient for further star formation on atimescale of around 1 . T . The diagonal character of the distribution of these galaxieson the { P, F } plane obviously points to the variability of SFR in galaxies of lowmasses. Facing periodic bursts, dIrr galaxies are moving from the top left to thebottom right quadrant, acquiring the signs of BCD galaxies. Note that Stinson etal. (2007) simulated the evolution of dIrr galaxies taking into account effects of gasoutflow due to the wind from SNe, and found cyclic bursts of star formation on thescale of ∼ . ∼ (2 − m for dwarf systems of very lowmasses.5. Scattering of the LV galaxies on the { P, F } diagram is quite high, reaching two to fourorders of magnitude depending on the morphological type. As we already noted, the Hα flux measurement error normally does not exceed ∼ . F ( Hα ) into [SFR], discussed by Pflamm–Altenburg & Kroupa (2009) also affects theparameter spread, but it shifts the galaxies exactly arriswise F = − P . Thus, much ofthe galaxy dispersion in Figure 6 does not have an instrumental origin, but a cosmicone. The smallest dispersion, σ ( P ) = 0 . , σ ( F ) = 0 .
6, is observed for the populationof late type-spirals, Sc–Sdm. It is most likely that the rotation of Sc–Sdm galaxiesand the stimulation of star formation it causes in gas-rich disks makes this processfairly regular.
8. The present cosmic SFR density
As demonstrated by Madau et al. (1996), Villar et al. (2008), Gonzalez et al. (2010),Westra et al. (2010), and other authors, the average SFR in previous epochs z ≃ − ρ SF R ( z ), it is important to reliably fix the current value of ρ SF R (0)from the observations of nearby galaxies.To this end, we used all available data on the SFR of galaxies situated within 8 Mpc atgalactic latitudes | b | > ◦ . We did not consider more distant objects because the presentcompleteness of the Hα survey drops appreciably beyond 8 Mpc. The integrated SFR forthe 8 Mpc sample amounts to 53 M ⊙ yr − . As is seen from Figure 1, the present Hα surveyis quite complete up to M B = − m . Based on the relation ”SFR versus M B ” in Figure4, we estimate that the integrated contribution of dwarf galaxies still unobserved in Hα
15 –adds about 4 M ⊙ yr − to the total amount. Therefore, the mean SFR density within 8 Mpcturns out to be ρ SF R (0) = 0 . M ⊙ yr − M pc − . As was noted by Karachentsev & Kut’kin(2005), the mean stellar mass density within 8 Mpc, estimated from the K-band luminositydensity j K ( L | M pc ) = 6 . × L ⊙ M pc − , appeared to be (1 . ± .
2) times higher thanthe mean cosmic density j K ( L ) cosmic = (3 . ± . × L ⊙ M pc − obtained by Cole et al.(2001) and Bell et al. (2003) from 2MASS. Reducing for the local overdensity, yields themean cosmic density of SFR in the present epoch ρ SF R (0) = (0 . ± . M ⊙ yr − M pc − .Table 6 gives a comparison of our estimate with the data obtained by other authorsbased on the samples of different depths and different compilation methods. As one cansee, the agreement of independent estimates of ρ SF R (0) is quite satisfactory.The data from Table 7, gathering the values of some basic cosmic parameters describingthe star formation within 1 Mpc at z=0 and h=0.72 can be useful to validate variousmodels of galaxy evolution. The rows of the table present: (1) — the critical density ofmatter, (2) — the luminosity density in the K -band (also evaluating the mean density ofstellar mass at M ∗ /L K = 1 M ⊙ /L ⊙ ), (3) — the mean density of hydrogen mass accordingto HIPASS (Zwaan et al. 2003), (4) — the mean density of SFR, and (5,6) — the meandensity of the dimensionless parameters P and F , derived from the quantities of rows(2)–(4) via Equations (4). The value ρ ( P ) = − .
17, actually averaged with the galaxymasses proportional to their K –luminosity, means that the current star formation rate in aunit volume is only 1.5 times lower than the average SFR in past epochs. This result lookssignificantly at odds with the notion that the characteristic SFR in the z ≃ z = 0. The other value, ρ ( F ) = − .
50, means thatan average unit volume has such a reserve of gas in the galaxies, which is able to maintainthe average present rate of star formation in them for another 4–5 Gyr. In other words, ouruniverse has already gone more than halfway in the history of transformation of gas intostars and is now being on the descending branch of this process immediately after the eraof peak intensity of star formation. It is needless to stress that this assertion is true onlyunder the condition that the bulk of gas is located in the volume of the galaxies themselves,rather than being distributed in the intergalactic space.
9. Concluding remarks
The program of our massive Hα survey of galaxies in the neighboring volume a radiusof 10 Mpc allows us to determine some basic cosmic parameters, characterizing the rateand resource of star formation in the local universe. An important prerequisite for this isan exclusion of deliberate selection in the choice of objects for the observational program bymorphological type and/or other features. A simple principle is evident here: the lower theselectivity of objects is for observations, the simpler the interpretation of the data obtainedwill be. The lack of accurate distance measurements to a part of nearby galaxies somewhat 16 –blurs this ideal situation. It should be noted, however, that positions of galaxies in thediagnostic diagram { P, F } (Figure 6) do not depend on the errors of distance finding.Keeping the E, S0, and dSph galaxies, which are not expected to have Hα emission, inour sample we found surprisingly that in some of them the process of star formation goeson at a fairly high rate. For example, spheroidal galaxies KDG61, DDO44 and KKR25indicate the presence of separate emission knots, in which a young stellar population isformed. The isolated E and S0 galaxies, NGC404, NGC855, and NGC4460, demonstrateactive Hα emission in their central regions, which probably indicates a constant inflow ofthe accreting intergalactic gas into these galaxies (Moiseev et al. 2010). There are reasonsto assume that a population of young semi-formed dwarf galaxies, similar to the HI-cloudsin the Virgo (122746.2 +013601) and CVnI (122043.4 +461233) clusters, or to the HIJASS(102100.2 +684200) and Leib (AGC219303) objects in the M81 and LeoI groups is locatedin the upper right quadrant of the diagnostic diagram (Figure 6). To clarify the natureof such objects with masses comparable to the masses of dwarf galaxies, much deeperobservations are needed with flux limit of F ( Hα ) ∼ − erg/cm sec and F ( HI ) ∼ − J y km/sec. Such observations would obviously require a significant amount of time on largetelescopes. This demand should be related to the predictions of modern scenarios of galaxyevolution. So far, the existing shallow Hα survey of nearby dwarf galaxies is successfullycompeting with another survey of these galaxies in the ultraviolet range on GALEX (Gil dePaz et al. 2003) due to weaker absorption in the Hα line and higher angular resolution.The work was supported by the Russian Foundation for Basic Research (projects10–02–00123, 09–02–90414-UKR-f-a and 10–02–92650-IND-a). References
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The open symbols indicatethe galaxies with only the upper limit of their SFR. The line corresponds to a constant SFRper unit luminosity. 22 –Fig. 5.— SFR vs. neutral hydrogen mass for 435 LV galaxies. The objects with an upperlimit of SFR or M HI are indicated by open symbols. The dashed line corresponds to a fixed SF R per unit hydrogen mass and the solid line traces the relationship SFR ∝ M . HI . 23 –Fig. 6.— LV galaxies of different morphological types on the diagnostic diagram “Past-Future”. The objects with an upper limit of SFR or M HI are shown by open symbols witharrows. 24 –Table 1: Basic contributions to the Hα survey of the LV galaxiesReference N LV SampleKennicutt & Kent, 1983 25 Spiral,IrregularKennicutt et al. 1989 14 Spiral,IrregularHunter et al. 1993 37 IrregularMiller & Hodge,1994 11 M81 groupYoung et al. 1996 16 Spiralvan Zee, 2000 15 Isolated irregularBell & Kennicutt, 2001 24 Spiral,IrregularGil de Paz et al. 2003 10 BCDJames et al. 2004 49 S0/a–ImHunter & Elmegreen, 2004 50 Im, BCDMeurer et al. 2006 10 HIPASS selectedEpinat et al. 2008 27 SpiralKennicutt et al. 2008 171 T > −
1, B < m , | b | > ◦ Bouchard et al. 2009 18 Sculptor and CenA groupsThis paper 207 All types 25 –Table 2: Galaxies in the IC342/Maffei complexName RA Dec
T V LG D MW M B lg M ( HI ) lg F ( Hα ) lg F c ( Hα ) lg( SF R )(2000.0) km s − Mpc mag M ⊙ erg/cm s erg/cm s M ⊙ /yrKKH5 010732.5+512625 10 304 4.26 − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − Hα Name RA Dec
T V LG D MW M B lg M ( HI ) lg F ( Hα ) lg F c ( Hα ) lg SF R (2000.0) km s − Mpc mag M ⊙ erg/cm s erg/cm s M ⊙ /yrN404 010926.9+354303 1 195 3.24 − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − Hα fluxesGalaxy log( F )This Zitrin & Kennicuttpaper Brosch 2008 et al. 2008A112521 − − − KK13 − − − KK14 − − − KK15 − − − IC1727 − − − ± .06N672 − − − ± .06U1281 − − − ± .07KK16 − − − KK17 − − − N784 − − − ± .04N855 − − − ± .04Maffei2 − − − ± .06UA86 − − − ± .07N1569 − − − ± .01UA92 − − − ± .03N1560 − − − ± .05UA105 − − − ± .03DDO47 − − − ± .06Table 5: Median parameters for different morphological samplesType ( T ) N P F
E,S0,dSph ( <
1) 20 − − − − − − − H = 72 km s − Mpc − , extinctioncorrected)log( ρ SF R ) Reference Note M ⊙ /yr/Mpc − ± − ± − ± − ± − ± − ± − ± Hα Local universe − ± Hα Local VolumeTable 7: Some cosmic density parametersParameter Quantity Reference ρ c · M ⊙ /Mpc Spergel et al. 2007 j K ( L ) 3.8 · L ⊙ /Mpc Cole et al. 2001,Bell et al. 2003 ρ ( HI ) 0.44 · M ⊙ /Mpc Zwaan et al. 2003 ρ ( SF R ) 0.019 M ⊙ /yr Mpc This paper ρ ( P ) − ρ ( F ) −−