29 research outputs found
Starburst galaxies
The infrared properties of star-forming galaxies, primarily as determined by the Infrared Astronomy Satellite (IRAS), are compared to X-ray, optical, and radio properties. Luminosity functions are reviewed and combined with those derived from optically discovered samples using 487 Markarian galaxies with redshifts and published IRAS 60 micron fluxes, and 1074 such galaxies in the Center for Astrophysics redshift survey. It is found that the majority of infrared galaxies which could be detected are low luminosity sources already known from the optical samples, but non-infrared surveys have found only a very small fraction of the highest luminosity sources. Distributions of infrared to optical fluxes and available spectra indicate that the majority of IRAS-selected galaxies are starburst galaxies. Having a census of starburst galaxies and associated dust allow severl important global calculations. The source counts are predicted as a function of flux limits for both infrared and radio fluxes. These galaxies are found to be important radio sources at faint flux limits. Taking the integrated flux to z = 3 indicates that such galaxies are a significant component of the diffuse X-ray background, and could be the the dominant component depending on the nature of the X-ray spectra and source evolution
The Most Luminous Starbursts in the Universe
A summary of starburst luminosities based on PAH features is given for 243
starburst galaxies with 0 < z < 2.5, observed with the Spitzer Infrared
Spectrograph. Luminosity vLv(7.7um) for the peak luminosity of the 7.7um PAH
emission feature is found to scale as log[vLv(7.7um)] = 44.63(+-0.09) +
2.48(+-0.28)log(1+z) for the most luminous starbursts observed. Empirical
calibrations of vLv(7.7um) are used to determine bolometric luminosity Lir and
the star formation rate (SFR) for these starbursts. The most luminous
starbursts found in this sample have log Lir = 45.4(+-0.3) +
2.5(+-0.3)log(1+z), in ergs per s, and the maximum star formation rates for
starbursts in units of solar masses per yr are log(SFR) = 2.1(+-0.3) +
2.5(+-0.3)log(1+z), up to z = 2.5. The exponent for pure luminosity evolution
agrees with optical and radio studies of starbursts but is flatter than
previous results based in infrared source counts. The maximum star formation
rates are similar to the maxima determined for submillimeter galaxies; the most
luminous individual starburst included within the sample has log Lir = 46.9,
which gives a SFR = 3400 solar masses per yr.Comment: Accepted for publication in The Astrophysical Journa
Surface Brightness of Starbursts at Low and High Redshifts
Observations in the rest frame ultraviolet from various space missions are
used to define the nearby starburst regions having the highest surface
brightness on scales of several hundred pc. The bright limit is found to be
6x10^-16 ergs/cm^2-s-A-arcsec^2 for rest frame wavelength of 1830 A. Surface
brightness in the brightest pixel is measured for 18 galaxies in the Hubble
Deep Field having z > 2.2. After correcting for cosmological dimming, we find
that the high redshift starbursts have intrinsic ultraviolet surface brightness
that is typically four times brighter than low redshift starbursts. It is not
possible to conclude whether this difference is caused by decreased dust
obscuration in the high redshift starburst regions or by intrinsically more
intense star formation. Surface brightness enhancement of starburst regions may
be the primary factor for explaining the observed increase with redshift of the
ultraviolet luminosity arising from star formation.Comment: accepted for publication in AJ; 11 pages text, 3 tables, 3 figures
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Evolution of the Most Luminous Dusty Galaxies
A summary of mid-infrared continuum luminosities arising from dust is given
for very luminous galaxies, Lir > 10^12 solar luminosities, with 0.005 < z <
3.2 containing active galactic nuclei (AGN), including 115 obscured AGN and 60
unobscured (type 1) AGN. All sources have been observed with the Spitzer
Infrared Spectrograph. Obscured AGN are defined as having optical depth > 0.7
in the 9.7 um silicate absorption feature and unobscured AGN show silicate in
emission. Luminosity vLv(8 um) is found to scale as (1+z)^2.6 to z = 2.8, and
luminosities vLv(8 um) are approximately 3 times greater for the most luminous
unobscured AGN. Total infrared luminosities for the most luminous obscured AGN,
Lir(AGN_obscured) in solar luminosities, scale as log Lir(AGN_obscured) =
12.3+-0.25 + 2.6(+-0.3)log(1+z), and for the most luminous unobscured AGN,
scale as log Lir(AGN1) = 12.6+-0.15 + 2.6(+-0.3)log(1+z), indicating that the
most luminous AGN are about 10 times more luminous than the most luminous
starbursts. Results are consistent with obscured and unobscured AGN having the
same total luminosities with differences arising only from orientation, such
that the obscured AGN are observed through very dusty clouds which extinct
about 50% of the intrinsic luminosity at 8 um. Both obscured and unobscured AGN
should be detected to z ~ 6 by Spitzer surveys with fv(24 um) > 0.3 mJy, even
without luminosity evolution for z > 2.5. By contrast, the most luminous
starbursts cannot be detected for z > 3, even if luminosity evolution continues
beyond z = 2.5.Comment: Includes corrected Figure 3, as publishe
Average Infrared Galaxy Spectra From Spitzer Flux Limited Samples
The mid-infrared spectroscopic analysis of a flux-limited sample of galaxies
with fv(24um) > 10 mJy is presented. Sources observed are taken from the
Spitzer First Look Survey (FLS) catalog and from the NOAO Deep Wide-Field
Survey region in Bootes (NDWFS). The spectroscopic sample includes 60 of the
100 sources in these combined catalogs having fv(24um) > 10 mJy. New spectra
from the Spitzer Infrared Spectrograph are presented for 25 FLS sources and for
11 Bootes AGN; these are combined with 24 Bootes starburst galaxies previously
published to determine the distribution of mid-infrared spectral
characteristics for the total 10 mJy sample. Sources have 0.01 < z < 2.4 and
41.8 < log vLv(15um) < 46.2 (ergs/s). Average spectra are determined as a
function of luminosity; lower luminosity sources (log vLv(15um) < 44.0) are
dominated by PAH features and higher luminosity sources (log vLv(15um) > 44.0)
are dominated by silicate absorption or emission. We find that a rest frame
equivalent width of 0.4um for the 6.2um PAH emission feature provides a well
defined division between lower luminosity, "pure" starbursts and higher
luminosity AGN or composite sources. Using the average spectra, fluxes fv(24um)
which would be observed with the Spitzer MIPS are predicted as a function of
redshift for sources with luminosities that correspond to the average spectra.
AGN identical to those in this 10 mJy sample could be seen to z = 3 with
fv(24um) > 1 mJy, but starbursts fall to fv(24um) < 1 mJy by z ~ 0.5. This
indicates that substantial luminosity evolution of starbursts is required to
explain the numerous starbursts found in other IRS results having fv(24um) > 1
mJy and z ~ 2.Comment: Accepted for publication in The Astrophysical Journa
Comparing Ultraviolet and Infrared-Selected Starburst Galaxies in Dust Obscuration and Luminosity
We present samples of starburst galaxies that represent the extremes
discovered with infrared and ultraviolet observations, including 25 Markarian
galaxies, 23 ultraviolet luminous galaxies discovered with GALEX, and the 50
starburst galaxies having the largest infrared/ultraviolet ratios. These
sources have z < 0.5 and cover a luminosity range of ~ 10^4. Comparisons
between infrared luminosities determined with the 7.7 um PAH feature and
ultraviolet luminosities from the stellar continuum at 153 nm are used to
determine obscuration in starbursts and dependence of this obscuration on
infrared or ultraviolet luminosity. A strong selection effect arises for the
ultraviolet-selected samples: the brightest sources appear bright because they
have the least obscuration. Obscuration correction for the ultraviolet-selected
Markarian+GALEX sample has the form log[UV(intrinsic)/UV(observed)] =
0.07(+-0.04)M(UV)+2.09+-0.69 but for the full infrared-selected Spitzer sample
is log[UV(intrinsic)/UV(observed)] = 0.17(+-0.02)M(UV)+4.55+-0.4. The relation
of total bolometric luminosity L_{ir} to M(UV) is also determined for
infrared-selected and ultraviolet-selected samples. For ultraviolet-selected
galaxies, log L_{ir} = -(0.33+-0.04)M(UV)+4.52+-0.69. For the full
infrared-selected sample, log L_{ir} = -(0.23+-0.02)M(UV)+6.99+-0.41, all for
L_{ir} in solar luminosities and M(UV) the AB magnitude at rest frame 153 nm.
These results imply that obscuration corrections by factors of two to three
determined from reddening of the ultraviolet continuum for Lyman Break Galaxies
with z > 2 are insufficient, and should be at least a factor of 10 for M(UV)
about -17, with decreasing correction for more luminous sources.Comment: accepted for publication in The Astrophysical Journa
Star Formation Rates for Starburst Galaxies from Ultraviolet, Infrared, and Radio Luminosities
Star formation rates (SFR) are compared as determined from mid-infrared 7.7
um PAH luminosity [SFR(PAH)], from 1.4 GHz radio luminosity [SFR(radio)], and
from far ultraviolet luminosity [SFR(UV)] for a sample of 287 starburst
galaxies with z < 0.5 having Spitzer IRS observations. The previously adopted
relation log [SFR(PAH)] = log [vLv(7.7 um)] - 42.57+-0.2, for SFR in solar
masses per year and vLv(7.7 um) the luminosity at the peak of the 7.7 um PAH
feature in ergs per s, is found to agree with SFR(radio). Comparing with
SFR(UV) determined independently from ultraviolet observations of the same
sources with the GALEX mission (not corrected for dust extinction), the median
log [SFR(PAH)/SFR(UV)] = 1.67, indicating that only 2% of the ultraviolet
continuum typically escapes extinction by dust within a starburst. The ratio
SFR(PAH)/SFR(UV) depends on infrared luminosity, with form log
[SFR(PAH)/SFR(UV)]= (0.53+-0.05)log Lir - 4.11+-0.18, for Lir in solar
luminosities. We also find that the ratio SFR(PAH)/SFR(UV) is similar to that
in infrared-selected starbursts for a sample of Markarian starburst galaxies
originally selected using optical classification, which implies that there is
no significant selection effect in SFR(PAH)/SFR(UV) using starburst galaxies
discovered by Spitzer. These results indicate that SFRs determined with
ultraviolet luminosities require dust corrections by a factor of ~ 10 for
typical local starbursts but this factor increases to > 700 for the most
luminous starbursts at z ~ 2.5. With this amount of extinction, the optical
magnitude of a starburst having fv(7.7 um) of 1 mJy should be V ~ 25.6.Comment: Final version as published, including minor corrections made in proo
Redshift Distribution of Extragalactic 24 micron Sources
We present the redshift distribution of a complete, unbiased sample of 24
micron sources down to fnu(24 micron) = 300 uJy (5-sigma). The sample consists
of 591 sources detected in the Bootes field of the NOAO Deep Wide-Field Survey.
We have obtained optical spectroscopic redshifts for 421 sources (71%). These
have a redshift distribution peaking at z~0.3, with a possible additional peak
at z~0.9, and objects detected out to z=4.5. The spectra of the remaining 170
(29%) exhibit no strong emission lines from which to determine a redshift. We
develop an algorithm to estimate the redshift distribution of these sources,
based on the assumption that they have emission lines but that these lines are
not observable due to the limited wavelength coverage of our spectroscopic
observations. The redshift distribution derived from all 591 sources exhibits
an additional peak of extremely luminous (L(8-1000 micron) > 3 x 10^{12} Lsun)
objects at z~2, consisting primarily of sources without observable emission
lines. We use optical line diagnostics and IRAC colors to estimate that 55% of
the sources within this peak are AGN-dominated. We compare our results to
published models of the evolution of infrared luminous galaxies. The models
which best reproduce our observations predict a large population of
star-formation dominated ULIRGs at z > 1.5 rather than the AGN-dominated
sources we observe.Comment: 21 pages, 11 figures; accepted for publication in Ap
Strong PAH Emission from z~2 ULIRGs
Using the Infrared Spectrograph on board the Spitzer Space Telescope, we
present low-resolution (64 < lambda / dlambda < 124), mid-infrared (20-38
micron) spectra of 23 high-redshift ULIRGs detected in the Bootes field of the
NOAO Deep Wide-Field Survey. All of the sources were selected to have 1) fnu(24
micron) > 0.5 mJy; 2) R-[24] > 14 Vega mag; and 3) a prominent rest-frame 1.6
micron stellar photospheric feature redshifted into Spitzer's 3-8 micron IRAC
bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs),
usually interpreted as signatures of star formation. The PAH features indicate
redshifts in the range 1.5 =1.96 and a dispersion
of 0.30. Based on local templates, these sources have extremely large infrared
luminosities, comparable to that of submillimeter galaxies. Our results confirm
previous indications that the rest-frame 1.6 micron stellar bump can be
efficiently used to select highly obscured starforming galaxies at z~2, and
that the fraction of starburst-dominated ULIRGs increases to faint 24 micron
flux densities. Using local templates, we find that the observed narrow
redshift distribution is due to the fact that the 24 micron detectability of
PAH-rich sources peaks sharply at z = 1.9. We can analogously explain the
broader redshift distribution of Spitzer-detected AGN-dominated ULIRGs based on
the shapes of their SEDs. Finally, we conclude that z~2 sources with a
detectable 1.6 micron stellar opacity feature lack sufficient AGN emission to
veil the 7.7 micron PAH band.Comment: accepted for publication in ApJ; references corrected in Section 3.2
and Figure